Acoustic instrument calibration is fundamental to the quantitative use of its data for
estimating aquatic resource abundance. Regular calibrations also allow instrument
performance to be monitored to detect changes due to the environment or component
dynamics, degradation, or failure.
This is the second ICES Cooperative Research Report (CRR) focussed on calibrations of
acoustic instruments. The first, CRR No. 144 (Foote et al., 1987), was published during
the era of analogue electronics more than a quarter of a century ago. Since then, not
only has the acoustic equipment improved vastly with digital electronics and signal
processing, but the techniques for applying them to studies of marine organisms have
both advanced and diversified. Motivating, facilitating, and expediting these developments
is the work of the Fisheries Acoustics, Science and Technology Working Group
(WGFAST) of the International Council for the Exploration of the Sea (ICES).
CRR No. 144 guided the fisheries acoustics community to uniformly apply the sphere
method to calibrate survey equipment, generally single-frequency, split-beam echosounders.
Today, surveys of fishery resources are conducted using a large variety of
acoustic instruments including, but not limited to, single-frequency, multifrequency,
single-beam, split-beam, broad bandwidth, and multibeam echosounders; side-scan
and scanning sonars; acoustic Doppler current profilers; and acoustic cameras. These
instruments differ in the ways in which they function, are utilized, and the types of
measurements they provide. In most cases, they also require different calibration techniques
for optimizing the accuracy and characterizing the precision of the measurements.
With technological innovation proceeding at an ever faster pace, the challenge to create
a comprehensive and practical guide to calibrating acoustic instruments is formidable.
Obviously, not all acoustic instrumentation and methods are addressed here. The ones
that are addressed are in various states of maturity. Therefore, the practical aims of this
CRR are to document (i) acoustic instruments currently used in fisheries research and
surveys, (ii) theoretical principles of calibrating these instruments, and (iii) methods
currently being practiced for a selection of commonly used instruments.
To meet these goals, the WGFAST formed the Study Group on Calibration of Acoustic
Instruments (SGCal) at its meeting in April 2009. The SGCal first met in San Diego, CA,
USA in April 2010 to outline the document. Some chapters were drafted intersessionally.
The SGCal met for the second time in Reykjavik, Iceland in May 2011 to collectively
review some draft chapters. The drafts were refined intersessionally and merged.
The draft CRR was collectively reviewed at meetings of the SGCal, in Pasaia, Spain in
April 2013 and in New Bedford, MA, USA in May 2014. Multiple independent reviewers
provided input, and the final editing was completed in 2014. The authors hope that
this CRR will be a valuable reference to both novice and experienced users of fishery
acoustic instruments, but recognize that it is a provisional guide that requires refinement
and update as the field continues to progress.

Description:

Instrument calibration involves the characterization of measurement accuracy (bias or
systematic error) and precision (variability or random error). Sampling with the calibrated
instrument involves additional systematic and random error (Demer, 2004).
Calibration accuracy is estimated and optimized by comparing measured and assumed
values for a standard, and correcting for the difference. The selection and characterization
of a calibration standard is, therefore, paramount to the accuracy of an instrument
calibration (Foote and MacLennan, 1984). Calibration precision is estimated by comparing
multiple measures of a standard. Importantly, the performance of an instrument
and thus its calibration accuracy and precision may change vs. time or the environment
(Demer and Hewitt, 1993; Brierley et al., 1998a; Nam et al., 2007). Therefore,
instruments should be calibrated frequently within the range of environments where
they are used to make calibrated measurements (Demer and Renfree, 2008). If this is
not possible, account should be made for any changes in the instrument or environment
that appreciably affect the calibration accuracy and precision.
This report includes general instruction and current best practices for calibrating a selection
of acoustic instruments commonly used to conduct fishery science and surveys.
It also describes some less developed protocols for other acoustic instruments. For
practical reasons, not all fishery acoustic instruments are included.